This invention relates to a composition for circuit connection used for electrically connecting up-and-down confronting circuits and bonding and fixing them, as well as to a circuit-connecting method using said composition and a connected structure of semiconductor chips.
As the size of electronic parts becomes smaller and their thickness becomes thinner, the circuit used therein has become higher in density and fineness. Since the existing solder and rubber connector are incapable of coping with connection of these minute circuits, anisotropically electroconductive adhesive and film (hereinafter referred to as "connecting materials") have become used largely in the recent time.
As one of the connecting materials, there is known an anisotropically electroconductive adhesive which contains electrically conductive particles in an insulating adhesive in an amount enough to form electroconductive route only in the direction of thickness when volume is decreased in the direction of thickness by applying a pressure.
In putting this adhesive to use, an adhesive material layer is provided between the confronting circuits and pressure is applied or heat and pressure are applied to the adhesive layer, whereby the up and down circuits are electrically connected and adjacent circuits are insulated and the circuits are bonded and fixed in this state.
As such an adhesive, a thermoplastic composition comprising styrene-butadiene-styrene block copolymer or the like as a base polymer has been used from the viewpoint of convenience at the time of use.
As the field to which the adhesive material is applied becomes wider, however, it has become desired to enlarge the use temperature range by improving heat resistance and to improve applicability to fine circuits and reliability by enhancing adhesive strength. Since such desires cannot be satisfied by the existing thermoplastic compositions, the use of curing reaction system has been attempted.
On the other hand, in the field of electroconductive materials, it has been known to use rigid materials such as carbon, nickel, thermally fusible metal and the like as a filler. Some of the inventors of the present invention previously proposed the use of an electroconductive filler prepared by coating the surface of a polymeric nuclear material with a metallic thin layer of which thermal expansion coefficient and elastic modulus are close to those of the adhesive used for connection of circuits (hereinafter, such a filler is referred to as "pressure-deformable electroconductive particle") (Japanese Patent Application No. 61-31088). According to this method, electroconductive material and adhesive undergo nearly equal expansion and shrinkage upon temperature change of connected part and therefore the electroconductive material satisfactorily follows the change in the gap between connected circuits so that the change in connection resistance is small. This, reliability of connection can be greatly improved as compared with the case using a rigid electroconductive material different from adhesive in thermal expansion coefficient and elastic modulus.
Although the use of a curing type adhesive as the adhesive constructing connecting part is very effective for improving heat resistance and adhesive strength, this method is disadvantageous in that the connected product shows great dispersion and is unstable with regard to the connection resistance between up and down circuits.
It has been found that the unstability of connection resistance is attributed to that a slow reaction progresses between adhesive and curing agent during storage of connecting material to decrease fluidity of the adhesive and thereby the insulating adhesive becomes unable to be sufficiently removed from surface of electroconductive particle or to that a curing reaction of adhesive takes place upon the heating or pressing to enhance viscosity of the adhesive before the conductive particles come into a sufficient contact with circuit and thereby the electroconductive particles and circuits are connected in a state of insufficient contact. It has been found that such an insufficient contact is particularly remarkable when the electroconductive particle is a pressure-deformable particle.
Another problem involved in curing reaction system is the difficulty to reuse insufficiently connected parts. Thus, in curing reaction system, the connected part is solidly bonded and the adhesive forms a network (crosslinkage), so that heating cannot bring about a sufficient decrease in bonding force and the cured adhesive is insoluble into solvent, which makes it quite difficult to peel off the insufficiently connected parts.
Accordingly, the connected part must be removed by a forcible means such as swelling and peeling by the use of solvent or chemical agent such as acid, alkali or the like or peeling-off by means of knife.
However, these forcible means injure the neighboring normally connected part and wirings, too, and a part of the adhesive inevitably remains on the surface, and therefore re-connection of high reliability is unexpectable.
It has been attempted to electrically connect semiconductor chips to circuit board by the use of the above-mentioned connecting materials.
For example, Japanese Patent Application Kokai (Laid-Open) No. 51-101469 disclosed a method for electrically connecting and bonding the bump provided on semiconductor chip to the connection terminal of the corresponding base board through intermediation of the electroconductive particles contained in anisotropic electroconductive adhesive, and Japanese Patent Application Kokai (laid-Open) No. 61-194731 disclosed a method for electrically connecting and bonding an electrode depressed from the surface of protecting layer provided on semiconductor chip to a bump provided on connecting terminal of circuit board by the use of an insulating adhesive or an anisotropic electroconductive adhesive by pressing and mutually contacting them.
However, at the present stage, these methods have not yet reached a level of practical mass production because of unsatisfactory reliability of connecting material.
The methods for electrically connecting a semiconductor chip to circuit board by the use of a connecting material disclosed in Japanese Patent Application Nos. 51-101469 and 61-194731 are quite excellent methods because they enable to realize a real connection by adhesive, and thereby they can cope with the tendency of decreasing the size of instruments and making circuits finer.
In these methods, however, a protruding or projecting electrode (bump) made of gold or solder and having a height of about 5 to 30 microns must be formed on at least one of the electrode of semiconductor chip and connection terminal of circuit board. Formation of uniform bumps on many fine circuits, generally having a line width of several to several tens microns, is disadvantageous in that it requires to provide a clean room of large scale and to use complicated high techniques such as formation of barrier metal layer, photolithography, plating, etching and the like, and it consumes a large quantity of expensive metals.
The main surface of semiconductor chip, not forming bump, is covered with an insulating material such as inorganic material (e.g. solicon oxide, boron nitride and the like) or polyimide type organic material (hereinafter, this layer of insulating material is referred to as "protecting layer"), and the electrode surface of semiconductor chip is usually provided concavely as compared with the level of this main surface.
Accordingly, for carrying out connection by the use of a connecting material such as anisotropic electroconductive adhesive or the like, a bump must be formed on at least one of the electrode part of semiconductor chip and connection terminal of circuit board as has been mentioned above. If no bump is formed on them (hereinafter, this state is referred to as "bumpless state"), connection has been impossible hitherto for the following reason. That is, in the electrode part, the electrode part is concave as compared with the level of main surface, so that the anisotropic electroconductive adhesive present in said part undergoes no pressure and its volume does not decrease even at the time of pressing for connection and hence no electricity-conducting route can be formed in the direction of thickness (from an electrode to an electrode). In the protecting layer part, pressure is concentrated into the contact part of particles, so that in case of usual rigid electroconductive particles breakage of semiconductor chip and protecting layer takes place.
In case of connecting a semiconductor chip having bump on main surface to a circuit base board, the following problem appears. That is, when a connecting material containing electroconductive particles such as anisotropic electroconductive adhesive is used, the connection at a small-area part such as bump is insufficient in reliability. The area of connection part in bump is usually as small as 100 micron square or less and the number of bumps on one chip is usually 100 or more, and the area of bump is more and more decreasing, aiming at higher fineness. If the amount of conductive particle is increased for the sake of improving the reliability of connection in minute area, short-circuit takes place between adjacent bumps. Thus, there has been a limit in the reliability of connection in minute area.
In addition, when the conductive particle is a rigid particle such as carbon or nickel particle, the pressure applied at the time of connection causes breakage of semiconductor chip due to stress or causes breakage in the protecting layer of circuit, and thereby the function of semiconductor chip is lost.
In case of connection using an insulating adhesive containing no conductive particle, connection is realized by the mutual contact of minute concavities and convexities, so that reliability of connection is deteriorated by the unevennesses in the flatness of circuit on board and bump height of semiconductor chip.